The present invention relates to a speed detecting apparatus, and more particularly to a speed detecting apparatus capable of detecting speeds ranging from low to high speeds with high accuracy.
DC and AC motors are controlled using data on the actual speeds of the motors fed back in a feedback loop. For example, in an AC motor control system in which the AC motor is driven by primary three-phase current commands generated by digital processing, the speed difference .DELTA.v between a commanded speed and an actual speed, and the actual speed n are supplied as inputs, and digital processing is carried out on the basis of .DELTA.v and n to determine the frequency and amplitude of the primary three-phase current commands. For example driving the spindle in a machine tool with an AC motor requires that the spindle be rotatable in a wide range of from low to high speeds. For accurate control, the speeds of rotation, both low and high, should be detected with precision.
According to a conventional speed detecting system, two-phase signals PA and PB which are .pi./2 out of phase with each other and have a frequency f proportional to the speed of rotation of the motor are generated, then the two-phase signals PA, PB are converter by a quadrupling circuit into signals having a frequency of 4f. Finally, a voltage (actual-speed voltage TSA) proportional to the speed of rotation is produced as an output of a frequency-to-voltage converter which serves to generate a voltage in proportion to the frequency 4f. With the prior system, however, as the pulse frequency becomes lower, the output voltage value from the frequency-to-voltage converter goes out of proportion to the frequency and is rapidly reduced. For this reason, the known system has not been suitable for the detection of speeds of AC motors which rotate at extremely low speeds.